Automatic frequency control system



c. TRAVIS 2,144,235 AUTOMATIC FREQUENCY CONTROL SYSTEM Filed Jan. 15,1957 2 Sheets-Sheet 1 Jan. 17, 1939.

TRAVIS P, P R i w h WA n" R m ES 852% g fifisfimm IF W $8 $3 4 J *4 \J\QI, U w W a u j? llllllll WM MP Q u n k v w u u g $39 3 1 :68 h x Em mum mugs w. W wfi N\| W EEEEwE QEEESE ATTORNEY Jan. 17, 1939; c. TRAVIS2,144,235 0 J I AUTOMATIC FREQUENCY CONTROL SYSTEM I Filed Jan. 15, 19372 Sheets-Sheet 2.

7'05/6M4L ...l m 01. EAMPL/FIE SOURCE 7' g Y I J Em ' 29 f2 LTOD/SCR/M/NATOR l TOD/SCR/M/NATOR 0/005 "-5 5 E I2 ro 0/005 INVENTORCHARLE TRAVIS ATTORNEY Patented Jan. 17, 1939 UNITED STATES PATENTOFFICE AUTOMATIC FREQUENCY CONTROL SYSTEM Charles Travis, Philadelphia,Pa., assignor to Radio Corporationof America, a corporation of DelawareApplicationJanuary 15, 1937, Serial No. 120,709

4 Claims.

arrangements for automatically adjustingthe frequency of the tunablelocal oscillator of a superheterodyne receiver in response to apredetermined shift in frequency of the I. F. energy from the assignedvalue thereof. In such systems the I. F. energy was impressed on a pairof rectifiers mistuned from the assigned I. F. by-

equal frequency amounts; 'and the direct current voltages produced byrectification were 'diiferentially combined, and employed to vary thegain of an electron discharge tube connected across the oscillator tankcircuit to function as areactance.

For various reasons it 'may be desirable to employ a push-pull controlcircuit in the frequency control network. For examplasuch a circuitwould be employed to avoid practical diffi- ,culties that arise inganging'the oscillator circuit and the control circuit when theoscillator and control circuit aretunable over a band, such gaugingbeing shown in Fig. 11 of my aforesaid application. In general,push-pull frequency control devices will require two separate biaschannels suchthat a change of frequency, or phase, at the discriminatorproduces approximately equal but opposite changes in 'the two biases.Each discriminator diode would, in other words, feeda separate biasline. The control circuit itself would comprise independent tubesarranged to produce positive or negative reactance effects acrosstheoscillator tank circuit depending on the action of the bias connectionsto the controltubes.

In other forms of the invention the local oscil- I later network may beconstructed to utilize the push-pull control biases. In such cases theoscillator generally includes a pair of oscillator 'tubecircuits tunedto slightly different frequencies, both combining to generateoscillations at a common mid-frequency; and the relative gains of thetwo oscillator circuit tubeswould then be varied by the push-pullcontrol bias.

Hence, it may be stated that the utilization of the afore-describedpush-pull frequency control circuits in'superheterodyne receivers, is amain objectof this application.

Another important object is to improve the action and efficiency ofautomatic frequency control (AFC) arrangements for superheterodynereceivers; the essential distinction over my aforesaid pendingapplication residing in the use of separate bias connections from thediscriminator rectifiersfor controlling the gain of each of a pair oftubes constructed and arranged to vary the oscillator network frequencyby a corrective frequency value when the I. F. energy shifts infrequency from its assigned magnitude.

Still other objects of the invention are generally to improve theoperation and reliability of automatic frequency control systems, andmore especially to provide a push-pull frequency control network whichis not only efiicient and reliable, but is economically, and readily,manufactured and assembled in superheterodyne receivers.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings:

Fig. 1 shows a frequency control circuit embodying one form of theinvention,

Fig. 2 illustrates a modification applied to the oscillator,

Fig. 3 shows a modified form of the invention illustrated in Fig. 2.

Referring now to the accompanying drawings, wherein like referencecharacters in the different figures denote similar circuit elements,there is shown in Fig. 1 that portion of a superheterodyne receiverlocated between the signal source and the second detector. Thoseskilled'in the art fully understand the manner of constructing thenetworks conventionally represented in Fig. 1. The signal source may bea grounded antenna circuit, a radio frequency distribution line, or anautomobile radio antenna. The tunable input circuit I of the firstdetector 2 is coupled through one, or more, tunable radio frequencyamplifiers to the source of signals. The variable condenser 3 may beadjusted to tune the circuit I over a wide signal frequency range; suchas 400 to 1600 k. c.; or through the short wave ranges. The localoscillator 4 is provided with a tunable tankcircuit 5; the. variablecondenser 6 thereof being adjustable through a range of frequenciesdiffering at all times from the signal the anodes of the diodes T1 andT2.

range by the operating I. F. The dotted line I denotes the usual tuningmechanism, which terminates in a manually adjustable device on thereceiver operating panel, mechanically coupling the rotors of thevarious variable condensers.

The I. F. energy produced by the first detector 2 is impressed upon one,or more, stages of I. F. amplification; the amplified energy is detectedin the usual second detector circuit. The audio voltage component ofdetected I. F. energy is amplified in one, or more, amplifiers, andfinally reproduced by any desired type of loudspeaker device. The I. F.may be chosen from a range of '75 to 450 k. c., and all resonantcircuits between the first detector and second detector will be tuned tothe selected I. F. value. Of course, any desired type of automaticvolume control circuit may be utilized; such volume control circuit maybe employed as shown in Fig. 1 of my aforesaid pending application. a

A portion of the I. F. energy, for example at the input to the seconddetector, is impressed upon the I. F amplifier II. In the plate circuitof amplifier 8 the primaries P1 and P2 of two similar transformers M4and Ms are connected in parallel and tuned to the exact center of the I.F. band by the condenser 9. The resonance curve of this compositeprimary is broadened by the resistor R, shunted across primary P1; theresistor may have a magnitude of 25,000 to 50,000 ohms. Thesecondariessi and S2 are respectively tuned equal increments above andbelow the I. F mid-band frequency.

Owing to the presence of resistor R across the common primary, thelatter is essentially a constant voltage source; and, as there is nodirect coupling between the two secondaries, the :total effectivecoupling between the latter is negligible. The secondaries each operateinto one of The cathodes of the diodes are connected in common to groundfor direct current. The load resistance I0, shunted by an I. F. by-passcondenser, is connected between the low potential side of the input coilS1 and the grounded cathode of diode T1. The load resistor II, shuntedby an I. F. by-pass condenser, is connected between the grounded cathodeof diode T2 and one side of the secondary S2. Of course the separatediodes may be replaced by a double diode rectifier having a commoncathode, such as a tube of the 6H6 type.

It is necessary to separate the resonant points of the two secondariesS1 and S2 by a minimum amount approximately equal to the I. F. midbandfrequency divided by the Q (or, ratio) of the circuits. For example, atan I. F. of 450 k. 0. without considering losses introduced by theprimary and by the diode load, a value to Q of 200 is about the highestthat can be obtained in the usual size of commercial I. F. coils. This.corresponds to a minimum separation of 2.25 k. c.

I I. Both connections I 2 and I3 include low pass filters which serve tosuppress the alternating current voltage components in the rectified I.F. energy. The numerals I2 and I3 denote the low pass filters insertedrespectively in connections I2 and I3. The tubes I3 and I4 may be of thewell-known pentode type.

The presence of a signal in the discriminator network, having afrequency in the I. F. range, will always produce negative biases onboth tubes I3 and I4. As the I. F. value changes one bias will becomenegative, and the other bias will become less negative. That is,approximately equal, but opposite, changes in AFC biases are produced asthe I. F. energy departs from its assigned frequency.

The tunable tank circuit 5 has its high alternating potential sideconnected to the plate of tube I3 through a condenser I5. The cathodelead of tube I3 is grounded, and the positive potential, for the plateof tube I3 is supplied through a choke coil I6. The plate of tube I4 isconnected to the high alternating potential side of tank circuit 5,through a series path which includes condenser I1 and coil I8; thepositive potential for the plate of tube I4 being applied through chokeI6. It will be noted that the low alternating potential side of the tankcircuit 5 is grounded, and the cathodes of tubes I3 and I4 are alsogrounded. It is to be understood that a common source of direct currentpotential may be employed for energizing the various electrodes of tubesI3 and I4, and this may well be the common direct current voltage supplyfor the rest of the receiver circuits.

It will now be seen that the oscillator tank circuit 5 is shunted by twoarms, one of them comprising the condenser I5 which is in series withthe variable tube resistance of tube I3; and the other arm comprisingthe inductance I8 in series with the variable tube resistance of tubeI4. The condenser I 1 functions as a low impedance blocking condenser inorder to separate the direct current potentials. The magnitude ofcondenser I5 and that of inductance I8 are so proportioned that thesquare root of the inductance of coil I8 divided by the capacitance ofcondenser I5 is equal to the mean value of the internal resistances oftubes I3 and I4, and this mean value may be expressed by the symbol Rp.Furthermore, condenser I5 and coil I8 should be chosen to resonate inthe middle of the band of frequency coverage. In other words, condenserI5 and coil I8 are designed to resonate in the middle of the frequencyrange of the tunable local oscillator tank circuit 5. In place of theselfinductance coil I8, the leakage inductance of a coil loosely coupledto the tank circuit coil may be used.

Under these conditions the variation in the internal resistances oftubes I3 and I4, which variation is caused by the direct current controlbiases derived from resistors I0 and II, will be approximately equal butopposite. The resistive part of the admittance thrown across the tankcircuit 5 will be constant with frequency and with variation in the twobiases (equal to Rp, the mean value), but the reactive component willvary as the internal resistance of tube I3 increases, while the internalresistance of tube I4 decreases, or vice versa. This method of pushpullfrequency control gives a close approximation to uniform oscillatoraction and uniform (percentage) control action over the tuning band.Considering the operation of the arrangement shown in' Fig; l morespecifically, it will'be seen thatif the received I." F. energy variesinfrequency from the assigned mid-band frequency, then there will bedeveloped an increasing direct current voltage across the diode loadresistor disposed in series with thedi scriminator diode inputcircuitwhich is tuned to the side of the frequency shift. For example, assumingthat the assigned I. Ffvalue is 450 k. c., then a shift in I.'F energyto 445 k. c. will cause direct current voltage to be developed acrossresistor Ill; assumingQthat the secondary Sris tuned to 445 k. c;

Whensuch mistuning occurs, the increased voltage developedacrossresistor Ill isapplied tothe'.

input grid of tube l3, and the internalresistance of the tube isincreased; this decreasing the effect of 'the capacitative arm acrosstank circuit 5.

This, in turn, reduces the effective capacity in the tankcircuit 5, andincreasees the frequency of the tank circuit. As a result, the frequencyof the local oscillations impressed'on the first detector increases,andthe' frequency value of the I. FQenergy increasesl' Of course, theconstants ofjth'e circuit are so chosen that the effect of the capacitypath acrossthe tank circuit5 is reduced to an extent sufficient to havethe I. F. energy outputof first detector 2 rise in frequency value i toapproximately 450 k. c. In this way it is posthe I. F. energy to a sibleto compensate for a shift in frequency of.

value of 450 k. c.

Under the assumed conditions, the secondary Szwould be tuned to 455k.c1; assuming nowthat the I. F. energy shifts in frequency to avalue of455 k. 0., direct current voltage will be produced across resistor II,and the internal resistance of tube l4 will be increased. This willreduce the shunt inductive effect of coil |8 across tank circuit 5, andcause the local oscillator tank circuit frequency to decrease.Thedecrease in oscillator frequency is sufficiently great 'tobring theoutput energy of first detector 2 back to approximately 450 k. c. Itwill, therefore, be appreciated thatby means of the separate controlbias lines 12 and I 3', and the capacity and inductance arms across tankcircuit 5, it is possible automatically to adjust the oscillator tankcircuit frequency to compensate for frequency shift of the I. F. energyaway from the assigned mid-band frequency. It will be realized that sucha frequency shift is not only due to thermal effects, as when thesuperheterodyne receiver is operating for a long period of time, butmay, also, be cause during the process of adjusting the tuning of thereceiver. With the control circuit disclosed it is possible to secureaccurate tuning, since the function of the discriminator and frequencycontrol unit is to pull the local oscillator into accurate tuningrelation with the incoming signal. Theoretically considered, there isprovided in Fig. 1 a frequency control unit which comprises anoscillator tank circuit having parallel capacity and inductive armsvalue below the assigned the relative gains of the two oscillatorcircuit tubes, which may be done by the use of the pushpull control biasarrangement described, willshift the generated frequency towards thenatural frequency of the circuit of the tube having the larger gain. Nowif the reactive coupling is weak, only a small frequency range ispossible, andif this coupling is increased difficulties will beencountered because of the fact that the common tank circuit has twodegrees of freedom of oscillatio-n. In such case, there would be twopossible operating frequencies and. drag loop" effects with sudden jumpsfrom one frequency tothe other will occur. 1 To avoid this last effect,it is proposed to keep the physical reactive coupling between the twotank circuits as small as need be, no. coupling being desirabl'efand tomake the operating frequencydependent upon electronic coupling. Thus, inFig. 2 there is shown an arrangement which is desirable in operation,and

embodies the action described. i

In Fig. 2 there is shown an arrangement embodying this form of frequencycontrol network.

The local oscillator network comprises a pair of tubes, of the pentodetype, having independent tunable circuits 5' and 5". and 5" are tuned toslightlydifferent frequencies, but each cooperates with'the other to generate a common mid-frequency oscillation which is used to heterodynewith the received signal frequency to produce the operating I. F. Theplates of tubes 20 and 2| are connected to a source of proper positivepotential through a common path 22 including ticklercoils 23 and 24 inSeries. The coil 23 is magnetically coupled, as at 25, to the tankcircuit 5"; the coil 24 is magnetically coupled, as at 26, to the tankcircuit 5'. Both tank circuits are coupled to the first detector networkto feed local oscillations thereto; 7

The uni control adjusting means I actuates thetuning condenser 3 andvariable condensers 21 and 2B. The AFC bias from the discriminatordiodes may be applied to the suppressor grids 29 and 30; leads l2 andI3, for example of Fig. 1, may be connected to the grids 29 and 30. Thecontrol biases may, also; be inserted at the screens, but this is not asdesirable since the screens draw current. If the plate impedances of-thetubes 20 and 2| are high, there will be little physical coupling betweenthe two tank circuit coils due to the fact that the tickler coils 23, 24are in series with a high impedance.

In Fig. 3 there is shown an alternative of the arrangement illustratedin Fig. 2. The tubes '30 and 3| are tubes of the pentode type. Thetunable tank circuit 32 is connected between the grid 33 and cathode oftube 3!], while the tank circuit 33 is connected between the grid 34 andcathode of tube 3|. The plate of tube 30 is magnetically coupled to thetank circuit 32 by the tickler coil 43, and the tickler coil 4|magnetically couples the plate of tube 3| to the tank circuit 33. Thesuppressor grid 50 of tube 30 is connected through the AFC connection l2to one of the discriminator diodes, and the grid 5|] is furthermoreconnected through condenser (ill and lead 6| to the high alternatingpotential side of tank circuit 33. The grid 10 of tube 3| is connectedthrough the AFC connection I3 to the second discriminator diode, andcondenser 80 and lead 8| connect the grid 10 to the high alternatingpotential side of tank circuit 32.

There is thus provided in Fig. 3 an oscillator network which comprisestwo oscillator tubes,

The tank circuits 5' each using one grid and the plate as the oscillatorelements, and the other grid as a means for injecting the voltage fromthe opposite oscillator tube; both grids being shielded by a positivescreen. In the circuit of Fig. 3 the plate and the grid 33 of tube 30are the oscillator elements; the grid 34 and the plate of tube 3! arethe oscillator elements of the latter tube. Looking in takes place byelectronic cross-coupling on the outer grids and 10. When the AFC biasapplied through lead I2 becomes more negative, and the bias appliedthrough lead I3 becomes less negative, the common mid-frequency isbrought nearer to the natural frequency of oscillator tank circuit 33.Conversely, when the bias applied to grid 50 becomes less negative, andthe bias applied to grid 10 becomes more negative, then the commonmid-frequency of the network is brought nearer to the natural frequencyof tank circuit As in the case of Fig. 2, the tank circuits 32 and 33are tuned to slightly different frequencies on either side of thepredetermined midfrequency.

Instead of using pentode tubes, it is possible to use tubes of the GA?type, and in such case the grid to which the AFC bias is applied wouldbe the outer, or fourth, grid. Furthermore, the functions of the gridsin each tube can be interchanged; the outer or fourth grid may functionas the oscillator element, and the locking-in action can be secured bymeans of the first, or inner, grid. In either of the circuits of Figs. 2or 3 it is possible so to gang the tank circuits that constant frequencycontrol sensitivity is produced over a tuning band, or possibly so thatany desirable variation of this sensitivity with mean frequency is to behad.

While I have indicated and'described several systems for carrying myinvention into effect, .it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

1. In a superheterodyne receiving system, a local oscillator networkincluding a tunable tank circuit, at least two reactive circuits ofopposite sign shunted across the tank circuit, and means,

responsive to a shift in the frequency of the intermediate energy from apredetermined assigned value, for automatically regulating the eifectsof said shunt circuits on the tank circuit.

2. In combination with the local oscillator and first detector of asuperheterodyne receiver, a pair of diode rectifiers connected to thefirst detector output circuit to rectify the intermediate energy outputof the detector, said oscillator including a tuned tank circuit, a pairof reactive arms of opposite sign in shunt with the oscillator tankcircuit, and separate bias connections from the diodes to said arms toregulate the eifectiveness of the arms in accordance with the relativemagnitudes of the outputs of said rectifiers.

3. In a superheterodyne receiver ofthe type including a converternetwork having an intermediate frequency output circuit and a localoscillator network provided with a tuned tank circuit, an automaticfrequency control arrange-,

ment including a discriminator network having an input circuit coupledto said output circuit, said discriminator having a direct currentvoltage output circuit producing two voltages in polarity opposition, apair of independent reactive circuits in shunt across said tank circuit,said shunt circuits each including reactances of opposite sign, a tubein each shunt circuit having its internal impedance in series with thereactance thereof, and means impressing each of said two voltages upon apredetermined tube in each shunt circuit for controlling the eifect ofthe re actances.

4. In combination with the local oscillator network of a superheterodynereceiver, said network

